“You hit it on the rock.”

His eyes widened. “The merfolk!”

“It tried to kill you.”

“It reached for my respirator…”

“It was trying to kill you, Marty. Then it tried to kill me. You pulled me out of the water. You saved my life—”

“I remember.” He looked around the small air pocket, assessing their dire predicament. “How long have we been here for?”

“I don’t know. A couple of hours, at least.”

“The air is still breathable.”

“I figure we might have two days’ worth.” She exhaled heavily. “You don’t know how relieved I am that you’re awake, Marty. I didn’t know what I was going to do. What are we going to do?” She bit her lip in frustration. Marty regaining consciousness might be a comforting development, but it hadn’t changed anything: they were still trapped and in peril.

Marty clenched his jaw. “I’m such a fool,” he muttered.

Elsa frowned. “What are you talking about?”

“I should have known. It was right there in front of my eyes. I refused to see it.”

“What the hell are you talking about, Marty?”

“I told you and everyone else that merfolk were likely mostly herbivorous.” He shook his head. “Did you see the merfolk’s teeth when it opened its mouth? They weren’t for gnawing down kelp and seaweed.”

“You couldn’t have known that. Both merfolk skulls were missing their teeth—”

“But I should have known. The sagittal crests meant they had powerful jaws—powerful jaws that were either needed to break down tough vegetation or to tear flesh from bone. I decided on the former explanation because it’s what I wanted to believe, because it reinforced my hypotheses that merfolk subsided on a largely plant-based diet similar to that of Great Apes and pre-fire hominins. But the evidence was pointing to the latter.”

“What evidence?”

“Both merfolk skulls that we’ve observed have been roughly the same size as a modern human’s.”

“Why’s that significant?”

“Because the size of an organism’s brain directly correlates with the amount and quality of energy it consumes. The human brain consumes twenty-five percent of our daily caloric intake. It wouldn’t have been possible for us to have evolved such a metabolically expensive organ eating a low-calorie, plant-based diet. Australopithecus had a cranial capacity roughly the size of a gorilla or chimpanzee. It was only when Homo erectus came onto the scene that brain sizes began increasing rapidly due to their mastery of fire and the advent of cooking.”

“Better nutrition, more calories, bigger brains…”